This invention relates generally to nonreciprocity of fiber-optic coils used in fiber-optic gyros and more specifically to methods and apparatus for determining pressure-induced nonreciprocity.
It was shown by D. M. Shupe in 1980 (D. M. Shupe, Thermally Induced Nonreciprocity in the Fiber-Optic Interferometer, Applied Optics, Vol. 19, No. 5, 1 Mar. 1980, pp. 654-655) that thermally-induced nonreciprocity in fiber-optic ring interferometers can occur if there are time-dependent temperature gradients along the fiber. If corresponding wavefronts of the counter-rotating beams traverse the same region of the fiber at different times, nonreciprocity arises. Shupe showed that this thermally-induced nonreciprocity results in an angular error .theta..sub.ET for the fiber-optic gyro given by ##EQU1## where where
n.sub.c is the refractive index of the fiber core; PA1 N is the number of turns in the fiber--optic coil; PA1 A is the area of the fiber--optic coil; PA1 L is the length of the fiber; PA1 T(t,l) is the temperature of the fiber element at a distance l from the end of the fiber and t is time; and PA1 .alpha. is the coefficient of linear thermal expansion of the fiber core.
In the case of a multilayer fiber coil wound on a cylindrical form such that the difference between the outside and inside coil diameter is small when compared with its average diameter and assuming that the coil temperature varies linearly from its inner layer to its outer layer, ##EQU2## and .DELTA.T is the change in the temperature difference across the coil from the initialization time 0 of the gyro to some later time t.
Fiber-optic coils are screened prior to assembly into fiber-optic gyros on the basis of measured thermally-induced angular error. The measurement of thermally-induced angular error of a fiber-optic coil is accomplished by applying a temperature ramp to the coil. The temperature ramp process is time-consuming and expensive since stabilization of the temperature depends on rather large coil time constants and reduction of angle random walk noise requires significant integration time. The measurement of thermally-induced angular error typically requires about 24 hours.
There is a need for a screening process which can be accomplished in a time much shorter than 24 hours.